The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
As mentioned above, the invention is particularly applicable to a lubrication system for a two-stroke internal combustion engine of a UAV. Accordingly, the invention will primarily be discussed in relation to that application. However, it should be understood that the invention may have application to various other machines, apparatus and devices having internal combustion engines which require lubrication.
The lubrication requirements for a two-stroke engine of a UAV are provided by way of a lubrication system comprising a pump for pumping lubrication oil to various parts of the engine, including for example the engine crankcase. The pump is connected to an oil supply, typically in the form of a reservoir such as a tank. The reservoir provides a supply of oil to the pump as required.
Typically, the pump is a positive displacement pump actuated by a solenoid.
It has been found that in such an engine lubrication system, the pump may not necessarily supply oil in a reliable manner to the engine in certain circumstances. This can lead to engine failure and may have catastrophic consequences for the UAV.
In particular, it has been found that in certain circumstances vapour cavities form in the oil. The vapour cavities may comprise air bubbles and voids arising from coalescence of air bubbles in the oil.
There are various reasons as to why vapour cavities may form in the oil, including agitation of the oil arising through, for example, operation of the UAV in turbulent flight conditions or at high altitude conditions in which ambient atmospheric pressure is relatively low such that the oil undergoes a phase change, and also cavitation associated with the pump.
The presence of vapour cavities in the oil can detrimentally affect operation of the pump, causing the supply of oil to the engine to be interrupted. In particular, the vapour cavities may block or at least retard flow of the oil into the pump. Furthermore, vapour cavities present within the pumping chamber of the pump may compress or collapse during a pump delivery stroke, thereby rendering the pump delivery stroke ineffective as it relies on volume compression of the otherwise incompressible oil.
It is against this background, and the problems and difficulties associated therewith that the present invention has been developed.